Support structure for MEMS device and methods therefor

ABSTRACT

A microelectromechanical systems device having support structures formed of sacrificial material surrounded by a protective material. The microelectromechanical systems device includes a substrate having an electrode formed thereon. Another electrode is separated from the first electrode by a cavity and forms a movable layer, which is supported by support structures formed of a sacrificial material.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims priority to U.S. patentapplication Ser. No. 11/491,389, filed Jul. 21, 2006, which claims thebenefit of U.S. Provisional Application No. 60/702,080, filed Jul. 22,2005. The disclosures of the foregoing applications are herebyincorporated by reference in their entireties.

BACKGROUND

1. Field

The field of the invention relates to microelectromechanical systems(MEMS). More specifically, the field of the invention relates tointerferometric modulators and methods of fabricating suchinterferometric modulators having supports for moving layers.

2. Description of the Related Technology

Microelectromechanical systems (MEMS) include micro mechanical elements,actuators, and electronics. Micromechanical elements may be createdusing deposition, etching, and/or other micromachining processes thatetch away parts of substrates and/or deposited material layers or thatadd layers to form electrical and electromechanical devices. One type ofMEMS device is called an interferometric modulator. As used herein, theterm interferometric modulator or interferometric light modulator refersto a device that selectively absorbs and/or reflects light using theprinciples of optical interference. In certain embodiments, aninterferometric modulator may comprise a pair of conductive plates, oneor both of which may be transparent and/or reflective in whole or partand capable of relative motion upon application of an appropriateelectrical signal. In a particular embodiment, one plate may comprise astationary layer deposited on a substrate and the other plate maycomprise a metallic membrane separated from the stationary layer by anair gap. As described herein in more detail, the position of one platein relation to another can change the optical interference of lightincident on the interferometric modulator. Such devices have a widerange of applications, and it would be beneficial in the art to utilizeand/or modify the characteristics of these types of devices so thattheir features can be exploited in improving existing products andcreating new products that have not yet been developed.

SUMMARY OF CERTAIN EMBODIMENTS

The system, method, and devices of the invention each have severalaspects, no single one of which is solely responsible for its desirableattributes. Without limiting the scope of this invention, its moreprominent features will now be discussed briefly. After considering thisdiscussion, and particularly after reading the section entitled“Detailed Description of Certain Embodiments” one will understand howthe features of this invention provide advantages over other displaydevices.

An embodiment provides a microelectromechanical systems device includinga substrate having a first electrode layer formed thereon, a secondelectrode, and at least one support structure. The second electrode isspaced apart from the first electrode layer by a cavity. The at leastone support structure has side surfaces surrounded by a protectivematerial preventing exposure of the side surfaces to the cavity. The atleast one support structure is configured to support the secondelectrode layer.

According to another embodiment, a method is provided for making ainterferometric modulator device. A substrate is provided. The substratehas a first electrode layer formed over the substrate. At least onesupport structure is surrounded with a protective material. A cavity iscreated between the first electrode layer and a movable layer aftersurrounding the at least one support structure.

According to yet another embodiment, an unreleased interferometricmodulator device is provided. The unreleased interferometric modulatordevice includes a substrate having a first electrode layer formedthereon, a sacrificial layer overlying the first electrode layer, and amovable layer. The sacrificial layer includes at least one supportstructure surrounded by a protective material over an entire side wallsurface of the support structure. A movable layer is positioned over thesacrificial layer.

In accordance with another embodiment, a method is provided for making amicroelectromechanical systems device. A substrate is provided. A firstelectrode layer is formed on the substrate. A sacrificial layer isdeposited over the first electrode layer. At least one annular hole isformed in the sacrificial layer and then filled. A second electrodelayer is deposited over the sacrificial layer after filling the annularhole. The sacrificial layer outside annular hole is removed afterdepositing the second electrode layer.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be readily apparent fromthe following description and from the appended drawings (not to scale),which are meant to illustrate and not to limit the invention, andwherein:

FIG. 1 is an isometric view depicting a portion of one embodiment of aninterferometric modulator display in which a movable reflective layer ofa first interferometric modulator is in a relaxed position and a movablereflective layer of a second interferometric modulator is in an actuatedposition.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device incorporating a 3×3 interferometric modulator display.

FIG. 3 is a diagram of movable mirror position versus applied voltagefor one exemplary embodiment of an interferometric modulator of FIG. 1.

FIG. 4 is an illustration of a set of row and column voltages that maybe used to drive an interferometric modulator display.

FIG. 5A illustrates one exemplary frame of display data in the 3×3interferometric modulator display of FIG. 2.

FIG. 5B illustrates one exemplary timing diagram for row and columnsignals that may be used to write the frame of FIG. 5A.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa visual display device comprising a plurality of interferometricmodulators.

FIG. 7A is a cross section of the device of FIG. 1.

FIG. 7B is a cross section of an alternative embodiment of aninterferometric modulator.

FIG. 7C is a cross section of another alternative embodiment of aninterferometric modulator.

FIG. 7D is a cross section of yet another alternative embodiment of aninterferometric modulator.

FIG. 7E is a cross section of an additional alternative embodiment of aninterferometric modulator.

FIGS. 8A-8C and 8E-8H are cross sections illustrating a sequence of aninterferometric modulator having support structures formed ofsacrificial material, in accordance with an embodiment.

FIG. 8D is a top view of an opening formed at the stage shown in FIG. 8Cin the sacrificial material of the embodiment shown in FIGS. 8A-8C and8E-8H.

FIG. 9 is a cross section of an interferometric modulator formed inaccordance with another embodiment.

FIGS. 10A, 10C, and 10D are cross sections illustrating a sequence offorming an interferometric modulator formed in accordance with anotherembodiment.

FIG. 10B is a cross section of an interferometric modulator formed inaccordance with yet another embodiment.

FIGS. 11A-11C are cross sections of an interferometric modulator formedin accordance with another embodiment.

FIGS. 12A, 12C, and 12E-12H are cross-sections illustrating certainsteps in the fabrication of an interferometric modulator in accordancewith yet another embodiment. FIGS. 12B, 12D, and 12I are top viewsillustrating the interferometric modulator at certain stages in thefabrication process of FIGS. 12A, 12C and 12E-12H.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

The following detailed description is directed to certain specificembodiments of the invention. However, the invention can be embodied ina multitude of different ways. In this description, reference is made tothe drawings wherein like parts are designated with like numeralsthroughout. As will be apparent from the following description, theembodiments may be implemented in any device that is configured todisplay an image, whether in motion (e.g., video) or stationary (e.g.,still image), and whether textual or pictorial. More particularly, it iscontemplated that the embodiments may be implemented in or associatedwith a variety of electronic devices such as, but not limited to, mobiletelephones, wireless devices, personal data assistants (PDAs), hand-heldor portable computers, GPS receivers/navigators, cameras, MP3 players,camcorders, game consoles, wrist watches, clocks, calculators,television monitors, flat panel displays, computer monitors, autodisplays (e.g., odometer display, etc.), cockpit controls and/ordisplays, display of camera views (e.g., display of a rear view camerain a vehicle), electronic photographs, electronic billboards or signs,projectors, architectural structures, packaging, and aestheticstructures (e.g., display of images on a piece of jewelry). MEMS devicesof similar structure to those described herein can also be used innon-display applications such as in electronic switching devices.

According to preferred embodiments, an interferometric modulator displayis provided with support structures (e.g., posts) formed of asacrificial material. The support structures are formed in the samelayer of material that serves as a sacrificial layer that is selectivelyetched to form the optical cavity of the interferometric modulator. Anannular opening is opened in a planar sacrificial layer and filled witha protective material. The protective material, and optionally anoverlying mechanical layer, prevent removal of the surroundedsacrificial material during the “release” etch of the sacrificialmaterial. The protected sacrificial material then serves as a supportfor the mechanical or movable layer, and the surrounding protectivematerial can remain in place or be subsequently removed.

One interferometric modulator display embodiment comprising aninterferometric MEMS display element is illustrated in FIG. 1. In thesedevices, the pixels are in either a bright or dark state. In the bright(“on” or “open”) state, the display element reflects a large portion ofincident visible light to a user. When in the dark (“off” or “closed”)state, the display element reflects little incident visible light to theuser. Depending on the embodiment, the light reflectance properties ofthe “on” and “off” states may be reversed. MEMS pixels can be configuredto reflect predominantly at selected colors, allowing for a colordisplay in addition to black and white.

FIG. 1 is an isometric view depicting two adjacent pixels in a series ofpixels of a visual display, wherein each pixel comprises a MEMSinterferometric modulator. In some embodiments, an interferometricmodulator display comprises a row/column array of these interferometricmodulators. Each interferometric modulator includes a pair of reflectivelayers positioned at a variable and controllable distance from eachother to form a resonant optical gap with at least one variabledimension. In one embodiment, one of the reflective layers may be movedbetween two positions. In the first position, referred to herein as therelaxed position, the movable reflective layer is positioned at arelatively large distance from a fixed partially reflective layer. Inthe second position, referred to herein as the actuated position, themovable reflective layer is positioned more closely adjacent to thepartially reflective layer. Incident light that reflects from the twolayers interferes constructively or destructively depending on theposition of the movable reflective layer, producing either an overallreflective or non-reflective state for each pixel.

The depicted portion of the pixel array in FIG. 1 includes two adjacentinterferometric modulators 12 a and 12 b. In the interferometricmodulator 12 a on the left, a movable reflective layer 14 a isillustrated in a relaxed position at a predetermined distance from anoptical stack 16 a, which includes a partially reflective layer. In theinterferometric modulator 12 b on the right, the movable reflectivelayer 14 b is illustrated in an actuated position adjacent to theoptical stack 16 b.

The optical stacks 16 a and 16 b (collectively referred to as opticalstack 16), as referenced herein, typically comprise several fusedlayers, which can include an electrode layer, such as indium tin oxide(ITO), a partially reflective layer, such as chromium, and a transparentdielectric. The optical stack 16 is thus electrically conductive,partially transparent, and partially reflective, and may be fabricated,for example, by depositing one or more of the above layers onto atransparent substrate 20. The partially reflective layer can be formedfrom a variety of materials that are partially reflective such asvarious metals, semiconductors, and dielectrics. The partiallyreflective layer can be formed of one or more layers of materials, andeach of the layers can be formed of a single material or a combinationof materials.

In some embodiments, the layers of the optical stack 16 are patternedinto parallel strips, and may form row electrodes in a display device asdescribed further below. The movable reflective layers 14 a, 14 b may beformed as a series of parallel strips of a deposited metal layer orlayers (orthogonal to the row electrodes of 16 a, 16 b) deposited on topof posts 18 and an intervening sacrificial material deposited betweenthe posts 18. When the sacrificial material is etched away, the movablereflective layers 14 a, 14 b are separated from the optical stacks 16 a,16 b by a defined gap 19. A highly conductive and reflective materialsuch as aluminum may be used for the reflective layers 14, and thesestrips may form column electrodes in a display device.

With no applied voltage, the gap 19 remains between the movablereflective layer 14 a and optical stack 16 a, with the movablereflective layer 14 a in a mechanically relaxed state, as illustrated bythe pixel 12 a in FIG. 1. However, when a potential difference isapplied to a selected row and column, the capacitor formed at theintersection of the row and column electrodes at the corresponding pixelbecomes charged, and electrostatic forces pull the electrodes together.If the voltage is high enough, the movable reflective layer 14 isdeformed and is forced against the optical stack 16. A dielectric layer(not illustrated in this Figure) within the optical stack 16 may preventshorting and control the separation distance between layers 14 and 16,as illustrated by pixel 12 b on the right in FIG. 1. The behavior is thesame regardless of the polarity of the applied potential difference. Inthis way, row/column actuation that can control the reflective vs.non-reflective pixel states is analogous in many ways to that used inconventional LCD and other display technologies.

FIGS. 2 through 5B illustrate one exemplary process and system for usingan array of interferometric modulators in a display application.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device that may incorporate aspects of the invention. In theexemplary embodiment, the electronic device includes a processor 21which may be any general purpose single- or multi-chip microprocessorsuch as an ARM, Pentium®, Pentium II®, Pentium III®, Pentium IV®,Pentium® Pro, an 8051, a MIPS®, a Power PC®, an ALPHA®, or any specialpurpose microprocessor such as a digital signal processor,microcontroller, or a programmable gate array. As is conventional in theart, the processor 21 may be configured to execute one or more softwaremodules. In addition to executing an operating system, the processor maybe configured to execute one or more software applications, including aweb browser, a telephone application, an email program, or any othersoftware application.

In one embodiment, the processor 21 is also configured to communicatewith an array driver 22. In one embodiment, the array driver 22 includesa row driver circuit 24 and a column driver circuit 26 that providesignals to a display array or panel 30. The cross section of the arrayillustrated in FIG. 1 is shown by the lines 1-1 in FIG. 2. For MEMSinterferometric modulators, the row/column actuation protocol may takeadvantage of a hysteresis property of these devices illustrated in FIG.3. It may require, for example, a 10 volt potential difference to causea movable layer to deform from the relaxed state to the actuated state.However, when the voltage is reduced from that value, the movable layermaintains its state as the voltage drops back below 10 volts. In theexemplary embodiment of FIG. 3, the movable layer does not relaxcompletely until the voltage drops below 2 volts. Thus, there exists awindow of applied voltage, about 3 to 7 V in the example illustrated inFIG. 3, within which the device is stable in either the relaxed oractuated state. This is referred to herein as the “hysteresis window” or“stability window.” For a display array having the hysteresischaracteristics of FIG. 3, the row/column actuation protocol can bedesigned such that during row strobing, pixels in the strobed row thatare to be actuated are exposed to a voltage difference of about 10volts, and pixels that are to be relaxed are exposed to a voltagedifference of close to zero volts. After the strobe, the pixels areexposed to a steady state voltage difference of about 5 volts such thatthey remain in whatever state the row strobe put them in. After beingwritten, each pixel sees a potential difference within the “stabilitywindow” of 3-7 volts in this example. This feature makes the pixeldesign illustrated in FIG. 1 stable under the same applied voltageconditions in either an actuated or relaxed pre-existing state. Sinceeach pixel of the interferometric modulator, whether in the actuated orrelaxed state, is essentially a capacitor formed by the fixed and movingreflective layers, this stable state can be held at a voltage within thehysteresis window with almost no power dissipation. Essentially nocurrent flows into the pixel if the applied potential is fixed.

In typical applications, a display frame may be created by asserting theset of column electrodes in accordance with the desired set of actuatedpixels in the first row. A row pulse is then applied to the row 1electrode, actuating the pixels corresponding to the asserted columnlines. The asserted set of column electrodes is then changed tocorrespond to the desired set of actuated pixels in the second row. Apulse is then applied to the row 2 electrode, actuating the appropriatepixels in row 2 in accordance with the asserted column electrodes. Therow 1 pixels are unaffected by the row 2 pulse, and remain in the statethey were set to during the row 1 pulse. This may be repeated for theentire series of rows in a sequential fashion to produce the frame.Generally, the frames are refreshed and/or updated with new display databy continually repeating this process at some desired number of framesper second. A wide variety of protocols for driving row and columnelectrodes of pixel arrays to produce display frames are also well knownand may be used in conjunction with the present invention.

FIGS. 4, 5A, and 5B illustrate one possible actuation protocol forcreating a display frame on the 3×3 array of FIG. 2. FIG. 4 illustratesa possible set of column and row voltage levels that may be used forpixels exhibiting the hysteresis curves of FIG. 3. In the FIG. 4embodiment, actuating a pixel involves setting the appropriate column to−V_(bia), and the appropriate row to +ΔV, which may correspond to −5volts and +5 volts, respectively Relaxing the pixel is accomplished bysetting the appropriate column to +V_(bias), and the appropriate row tothe same +ΔV, producing a zero volt potential difference across thepixel. In those rows where the row voltage is held at zero volts, thepixels are stable in whatever state they were originally in, regardlessof whether the column is at +V_(bias), or −V_(bias). As is alsoillustrated in FIG. 4, it will be appreciated that voltages of oppositepolarity than those described above can be used, e.g., actuating a pixelcan involve setting the appropriate column to +V_(bias), and theappropriate row to −ΔV. In this embodiment, releasing the pixel isaccomplished by setting the appropriate column to −V_(bias), and theappropriate row to the same −ΔV, producing a zero volt potentialdifference across the pixel.

FIG. 5B is a timing diagram showing a series of row and column signalsapplied to the 3×3 array of FIG. 2 which will result in the displayarrangement illustrated in FIG. 5A, where actuated pixels arenon-reflective. Prior to writing the frame illustrated in FIG. 5A, thepixels can be in any state, and in this example, all the rows are at 0volts, and all the columns are at +5 volts. With these applied voltages,all pixels are stable in their existing actuated or relaxed states.

In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) areactuated. To accomplish this, during a “line time” for row 1, columns 1and 2 are set to −5 volts, and column 3 is set to +5 volts. This doesnot change the state of any pixels, because all the pixels remain in the3-7 volt stability window. Row 1 is then strobed with a pulse that goesfrom 0, up to 5 volts, and back to zero. This actuates the (1,1) and(1,2) pixels and relaxes the (1,3) pixel. No other pixels in the arrayare affected. To set row 2 as desired, column 2 is set to −5 volts, andcolumns 1 and 3 are set to +5 volts. The same strobe applied to row 2will then actuate pixel (2,2) and relax pixels (2,1) and (2,3). Again,no other pixels of the array are affected. Row 3 is similarly set bysetting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3strobe sets the row 3 pixels as shown in FIG. 5A. After writing theframe, the row potentials are zero, and the column potentials can remainat either +5 or −5 volts, and the display is then stable in thearrangement of FIG. 5A. It will be appreciated that the same procedurecan be employed for arrays of dozens or hundreds of rows and columns. Itwill also be appreciated that the timing, sequence, and levels ofvoltages used to perform row and column actuation can be varied widelywithin the general principles outlined above, and the above example isexemplary only, and any actuation voltage method can be used with thesystems and methods described herein.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa display device 40. The display device 40 can be, for example, acellular or mobile telephone. However, the same components of displaydevice 40 or slight variations thereof are also illustrative of varioustypes of display devices such as televisions and portable media players.

The display device 40 includes a housing 41, a display 30, an antenna43, a speaker 45, an input device 48, and a microphone 46. The housing41 is generally formed from any of a variety of manufacturing processesas are well known to those of skill in the art, including injectionmolding and vacuum forming. In addition, the housing 41 may be made fromany of a variety of materials, including, but not limited to, plastic,metal, glass, rubber, and ceramic, or a combination thereof. In oneembodiment, the housing 41 includes removable portions (not shown) thatmay be interchanged with other removable portions of different color, orcontaining different logos, pictures, or symbols.

The display 30 of exemplary display device 40 may be any of a variety ofdisplays, including a bi-stable display, as described herein. In otherembodiments, the display 30 includes a flat-panel display, such asplasma, EL, OLED, STN LCD, or TFT LCD as described above, or anon-flat-panel display, such as a CRT or other tube device, as is wellknown to those of skill in the art. However, for purposes of describingthe present embodiment, the display 30 includes an interferometricmodulator display, as described herein.

The components of one embodiment of exemplary display device 40 areschematically illustrated in FIG. 6B. The illustrated exemplary displaydevice 40 includes a housing 41 and can include additional components atleast partially enclosed therein. For example, in one embodiment, theexemplary display device 40 includes a network interface 27 thatincludes an antenna 43, which is coupled to a transceiver 47. Thetransceiver 47 is connected to a processor 21, which is connected toconditioning hardware 52. The conditioning hardware 52 may be configuredto condition a signal (e.g., filter a signal). The conditioning hardware52 is connected to a speaker 45 and a microphone 46. The processor 21 isalso connected to an input device 48 and a driver controller 29. Thedriver controller 29 is coupled to a frame buffer 28 and to an arraydriver 22, which in turn is coupled to a display array 30. A powersupply 50 provides power to all components as required by the particularexemplary display device 40 design.

The network interface 27 includes the antenna 43 and the transceiver 47so that the exemplary display device 40 can communicate with one or moredevices over a network. In one embodiment, the network interface 27 mayalso have some processing capabilities to relieve requirements of theprocessor 21. The antenna 43 is any antenna known to those of skill inthe art for transmitting and receiving signals. In one embodiment, theantenna transmits and receives RF signals according to the IEEE 802.11standard, including IEEE 802.11(a), (b), or (g). In another embodiment,the antenna transmits and receives RF signals according to the BLUETOOTHstandard. In the case of a cellular telephone, the antenna is designedto receive CDMA, GSM, AMPS, or other known signals that are used tocommunicate within a wireless cell phone network. The transceiver 47pre-processes the signals received from the antenna 43 so that they maybe received by and further manipulated by the processor 21. Thetransceiver 47 also processes signals received from the processor 21 sothat they may be transmitted from the exemplary display device 40 viathe antenna 43.

In an alternative embodiment, the transceiver 47 can be replaced by areceiver. In yet another alternative embodiment, the network interface27 can be replaced by an image source, which can store or generate imagedata to be sent to the processor 21. For example, the image source canbe a digital video disc (DVD) or a hard-disc drive that contains imagedata, or a software module that generates image data.

The processor 21 generally controls the overall operation of theexemplary display device 40. The processor 21 receives data, such ascompressed image data from the network interface 27 or an image source,and processes the data into raw image data or into a format that isreadily processed into raw image data. The processor 21 then sends theprocessed data to the driver controller 29 or to the frame buffer 28 forstorage. Raw data typically refers to the information that identifiesthe image characteristics at each location within an image. For example,such image characteristics can include color, saturation, and gray-scalelevel.

In one embodiment, the processor 21 includes a microcontroller, CPU, orlogic unit to control operation of the exemplary display device 40. Theconditioning hardware 52 generally includes amplifiers and filters fortransmitting signals to the speaker 45, and for receiving signals fromthe microphone 46. The conditioning hardware 52 may be discretecomponents within the exemplary display device 40, or may beincorporated within the processor 21 or other components.

The driver controller 29 takes the raw image data generated by theprocessor 21 either directly from the processor 21 or from the framebuffer 28 and reformats the raw image data appropriately for high speedtransmission to the array driver 22. Specifically, the driver controller29 reformats the raw image data into a data flow having a raster-likeformat, such that it has a time order suitable for scanning across thedisplay array 30. Then the driver controller 29 sends the formattedinformation to the array driver 22. Although a driver controller 29,such as a LCD controller, is often associated with the system processor21 as a stand-alone Integrated Circuit (IC), such controllers may beimplemented in many ways. They may be embedded in the processor 21 ashardware, embedded in the processor 21 as software, or fully integratedin hardware with the array driver 22.

Typically, the array driver 22 receives the formatted information fromthe driver controller 29 and reformats the video data into a parallelset of waveforms that are applied many times per second to the hundredsand sometimes thousands of leads coming from the display's x-y matrix ofpixels.

In one embodiment, the driver controller 29, array driver 22, anddisplay array 30 are appropriate for any of the types of displaysdescribed herein. For example, in one embodiment, the driver controller29 is a conventional display controller or a bi-stable displaycontroller (e.g., an interferometric modulator controller). In anotherembodiment, the array driver 22 is a conventional driver or a bi-stabledisplay driver (e.g., an interferometric modulator display). In oneembodiment, the driver controller 29 is integrated with the array driver22. Such an embodiment is common in highly integrated systems such ascellular phones, watches, and other small area displays. In yet anotherembodiment, the display array 30 is a typical display array or abi-stable display array (e.g., a display including an array ofinterferometric modulators).

The input device 48 allows a user to control the operation of theexemplary display device 40. In one embodiment, the input device 48includes a keypad, such as a QWERTY keyboard or a telephone keypad, abutton, a switch, a touch-sensitive screen, or a pressure- orheat-sensitive membrane. In one embodiment, the microphone 46 is aninput device for the exemplary display device 40. When the microphone 46is used to input data to the device, voice commands may be provided by auser for controlling operations of the exemplary display device 40.

The power supply 50 can include a variety of energy storage devices asare well known in the art. For example, in one embodiment, the powersupply 50 is a rechargeable battery, such as a nickel-cadmium battery ora lithium ion battery. In another embodiment, the power supply 50 is arenewable energy source, a capacitor, or a solar cell including aplastic solar cell, and solar-cell paint. In another embodiment, thepower supply 50 is configured to receive power from a wall outlet.

In some embodiments, control programmability resides, as describedabove, in a driver controller which can be located in several places inthe electronic display system. In some embodiments, controlprogrammability resides in the array driver 22. Those of skill in theart will recognize that the above-described optimizations may beimplemented in any number of hardware and/or software components and invarious configurations.

The details of the structure of interferometric modulators that operatein accordance with the principles set forth above may vary widely. Forexample, FIGS. 7A-7E illustrate five different embodiments of themovable reflective layer 14 and its supporting structures. FIG. 7A is across section of the embodiment of FIG. 1, where a strip of metalmaterial 14 is deposited on orthogonally extending supports 18. Thesupports 18 can comprise isolated posts or continuous walls. Forexample, the supports 18 can include linear rails that support crossingstrips of mechanical or movable material, and/or isolated posts. In oneexample, rails provide primarily support and posts within each cavityserve to stiffen the mechanical layer.

In FIG. 7B, the moveable reflective layer 14 is attached to supports atthe corners only, on tethers 32. In FIG. 7C, the moveable reflectivelayer 14 is suspended from a deformable layer 34, which may comprise aflexible metal. The deformable layer 34 connects, directly orindirectly, to the substrate 20 around the perimeter of the deformablelayer 34. These connections are herein referred to as support posts. Theembodiment illustrated in FIG. 7D has support post plugs 42 upon whichthe deformable layer 34 rests. The movable reflective layer 14 remainssuspended over the gap, as in FIGS. 7A-7C, but the deformable layer 34does not form the support posts by filling holes between the deformablelayer 34 and the optical stack 16. Rather, the support posts are formedof a planarization material, which is used to form support post plugs42. The embodiment illustrated in FIG. 7E is based on the embodimentshown in FIG. 7D, but may also be adapted to work with any of theembodiments illustrated in FIGS. 7A-7C, as well as additionalembodiments not shown. In the embodiment shown in FIG. 7E, an extralayer of metal or other conductive material has been used to form a busstructure 44. This allows signal routing along the back of theinterferometric modulators, eliminating a number of electrodes that mayotherwise have had to be formed on the substrate 20.

In embodiments such as those shown in FIG. 7, the interferometricmodulators function as direct-view devices, in which images are viewedfrom the front side of the transparent substrate 20, the side oppositeto that upon which the modulator is arranged. In these embodiments, thereflective layer 14 optically shields the portions of theinterferometric modulator on the side of the reflective layer oppositethe substrate 20, including the deformable layer 34. This allows theshielded areas to be configured and operated upon without negativelyaffecting the image quality. Such shielding allows the bus structure 44in FIG. 7E, which provides the ability to separate the opticalproperties of the modulator from the electromechanical properties of themodulator, such as addressing and the movements that result from thataddressing. This separable modulator architecture allows the structuraldesign and materials used for the electromechanical aspects and theoptical aspects of the modulator to be selected and to functionindependently of each other. Moreover, the embodiments shown in FIGS.7C-7E have additional benefits deriving from the decoupling of theoptical properties of the reflective layer 14 from its mechanicalproperties, which are carried out by the deformable layer 34. Thisallows the structural design and materials used for the reflective layer14 to be optimized with respect to the optical properties, and thestructural design and materials used for the deformable layer 34 to beoptimized with respect to desired mechanical properties.

Layers, materials, and/or other structural elements may be describedherein as being “over,” “above,” “between,” etc. in relation to otherstructural elements. As used herein, these terms can mean directly orindirectly on, over, above, between, etc., as a variety of intermediatelayers, material, and/or other structural elements can be interposedbetween structural elements described herein. Similarly, structuralelements described herein, such as substrates or layers, can comprise asingle component (e.g., a monolayer) or a multi-component structure(e.g., a laminate comprising multiple layers of the recited material,with or without layers of additional materials). Use of the term “one ormore” with respect to an object or element does not, in any way,indicate the absence of a potential plural arrangement of objects orelements for which the term is not used. The term“microelectromechanical device,” as used herein, refers generally to anysuch device at any stage of manufacture.

FIGS. 8A-8C and 8E-8H are cross-sectional views, illustrating anexemplary method of forming an interferometric modulator having supportstructures that are protected from subsequent processing. For example, aprotective liner can protect the support structures from a “release”etch that forms the MEMS cavity. In this instance, the supportstructures can be formed from the same sacrificial material that fillsthe cavity prior to the release etch.

According to this embodiment, an optical stack 16, such as the onesshown in FIGS. 7A-7E, is formed on a transparent substrate 20. Asdiscussed above, the optical stack 16 typically comprises severalintegrated or fused layers, including a first electrode layer 110, suchas ITO, a partially reflective layer 120, such as chromium, and adielectric layer 130. The layers of the optical stack 16 are preferablypatterned into parallel strips to form row electrodes. Typically, asshown in FIG. 8A, the layers of the optical stack 16 are deposited ontoa transparent substrate 20, preferably deposited by conventionaldeposition techniques, such as, for example, some form of sputtering,physical vapor deposition (PVD), chemical vapor deposition (CVD),spin-on dielectric (SOD) and spin-on glass (SOG). The dielectric layer130 of the optical stack 16 is preferably formed of silicon dioxide(SiO₂). In other arrangements, the dielectric layer 130 is formed ofother insulating materials and can optionally include one or more etchstop layers to protect the optical stack 16 from subsequent etch steps.As is understood by the skilled artisan, etch stop layers are highlyresistant to certain etching techniques to protect the material overwhich it is formed. Suitable materials for etch stop layers include, forexample, Al₂O₃, titanium, tungsten, amorphous silicon, germanium, andcombinations thereof.

According to this embodiment, formation of the optical stack 16 isfollowed by deposition of a sacrificial material 140 over the dielectriclayer 130, as shown in FIG. 8B. The sacrificial material 140 ispreferably deposited (and later selectively removed) over the opticalstack 16 to define a resonant optical cavity 180 (FIG. 8H) between theoptical stack 16 and the movable layer 170 (FIG. 8H) that will bedeposited over the sacrificial material 140. The thickness of thesacrificial material 140 is thus selected for a particular choice ofreflected color in the relaxed condition. In other arrangements,multiple thicknesses are deposited to produce multiple different colors,such as red, green, and blue for an RGB display system. In an exemplaryembodiment, a modulator having a cavity with the largest height (formedby a sacrificial layer having the greatest thickness) reflects redlight, a modulator having a cavity with an intermediate height (formedby a sacrificial layer having an intermediate thickness) reflects greenlight, and a modulator having a cavity with the smallest height (formedby a sacrificial layer having the smallest thickness) reflects bluelight. Though not separately illustrated, the skilled artisan willreadily appreciated that the post formation techniques disclosed hereinare equally applicable to monochromatic devices and multicolorinterferometric modulators with multiple different thicknesses ofsacrificial materials.

The sacrificial material 140 preferably comprises an inorganic material.Organic support structures have been provided in interferometricmodulator displays because the organic support structure can be made tobe very co-planar with the sacrificial layer. However, the skilledartisan will appreciate that, in interferometric modulators, organicsupport structures can lead to color shift and image retention problemsassociated with thermal stability and creep. Thus, inorganic supportstructures that can be made to be co-planar with the sacrificial layerwould help to minimize the color shift and image retention problems ofthe organic support structures. However, the skilled artisan willappreciate that the sacrificial material 140 can comprise an organicmaterial. If the sacrificial material 140 is organic, O₂ plasma or downstream O₂ can be used to remove the organic sacrificial material 140.

According to one embodiment, the sacrificial material 140 may comprisesilicon (Si). In other embodiments, this sacrificial material 140 may beformed of a metal, such as molybdenum (Mo), tungsten (W), titanium (Ti),and tantalum (Ta). In yet another embodiment, the sacrificial material140 may be formed of silicon-rich silicon nitride. All of thesesacrificial materials can be selectively etched by fluorine-basedetchants, relative to the exposed dielectric and electrode materials,but the skilled artisan will readily appreciate that other sacrificialmaterials can be used with other selective etch chemistries.Advantageously, silicon and the metals listed above provide more robustsupport than photoresist, and are particularly susceptible to selectiveetching by exposure to XeF₂.

As shown in FIG. 8C, in this embodiment, interferometric modulatorstructure production is continued by patterning and etching openings 150in the sacrificial material 140. The openings 150 are preferably in theform of annular holes, as shown in FIG. 8D, which is a top view of anopening 150. While the illustrated shape is round, the skilled artisanwill understand that the openings may be formed in other enclosed orcontinuous shapes to surround a portion of the sacrificial material 140that will form a support structure, including posts and elongated rails,of the device, as will described in more detail below. It will beunderstood that, as illustrated in FIG. 8C, the widths of the grooves ofthe openings 150 is much smaller than the diameter or width of thesupport structure to be formed. However, the skilled artisan willappreciate that for support structures having a smaller diameter orwidth (e.g., <4-5 microns), the widths of the grooves of the openings150 will be substantially on the same order as the diameter or width ofthe support structures.

As illustrated in FIG. 8E, a protective material 160 is deposited overthe structure to fill the openings 150 with the protective material 160.Suitable materials for the protective material include, but are notlimited to, self-planarizing material, such as photoresist, spin-ondielectric (SOD), etc. These materials are self-planarizing as they aredeposited by a spin-on deposition process. Moreover, both photoresistmaterials and silicon oxide based SODs are resistant to fluorine-basedetchants. Other suitable materials for the protective material 160include photo-sensitive polyimide, color pigment for color filterapplication, BCB (benzocyclobutene, commercially available from DowChemical Co. of Midland, Mich.), and spin-on low-k materials. As shownin FIG. 8E, as a result of this deposition, a layer of the protectivematerial 160 is formed over the structure in addition to filling theopenings 150. The skilled artisan will appreciate that a protectivematerial 160 formed of a photoreactive polymer, such as photoresist, canbe irradiated, rendering the protective material 160 resistant toconditions suitable for etching, ashing, or otherwise removing thesacrificial material 140, which will be described in more detail below.

The protective material 160 and the sacrificial material 140 arepreferably selected such that the sacrificial material 140 isselectively and/or preferentially etchable over the protective material160. Selectivity is such that the protective material 160 will not wearaway to expose protected portion (which will form the support structure)of the sacrificial material 140 over the course of a release etch (whichdepends on the geometry, etchant, materials, etc.). The sacrificialmaterial 140 is selectively or preferentially etchable relative to theprotective material 160 if an etchant can etch the sacrificial material140 at a substantially greater rate than the protective material 160(e.g., at a rate of greater than about 5×, preferably greater than about10×, and more preferably greater than about 40× the rate of etching ofthe protective material 160). The protective material 160 is thussubstantially resistant to etching under conditions under which thesacrificial material 140 is substantially susceptible to etching. Thoseskilled in the art will understand that the selection of the sacrificialmaterial 140 and the protective material 160 will depend on a variety offactors, including the methods and conditions used to deposit thematerials (which can affect the physical and/or chemical properties ofthe materials), and the etching conditions (including the nature of theetching process and the particular etchant used) during removal of thesacrificial material. Those skilled in the art will also understand thatall materials are etchable under the appropriate conditions and that thedescription herein of a material as selectively or preferentiallyetchable or etch resistant is in comparison with other materials presentin the device under the particular conditions to which the materials areexposed. Thus, in many instances, the selection of a sacrificialmaterial that is selectively or preferentially etchable relative to aprotective material is determined empirically, under controlledconditions. Alternatively, a wide variety of etching methods, systems,and materials that provide for selective etching of materials ofinterest are known in the art and/or commercially available. The skilledartisan will understand that the protective material 160 may bedeposited by various methods well known in the art, including, forexample, CVD, PVD, and sputtering.

The skilled artisan will appreciate that the thickness of the layer ofthe protective material 160 (which is selected by the dimensions of theannular opening 150) depends on the thickness of the sacrificialmaterial 140. For example, if the sacrificial material 140 is thicker,then the layer of protective material 160 is preferably also thicker. Asmentioned above, the thickness of the sacrificial material 140 willaffect the colors produced and that the colors produced also depend onother material properties of the sacrificial material 140. According toa preferred embodiment, the layer of protective material 160 has athickness that is about ½ to twice the thickness of the sacrificialmaterial 140. The layer of protective material 160 preferably has athickness (from the top surface of the sacrificial material 140 to thetop surface of the protective material 160) in the range of 500 Å to onemicron, more preferably in the range of 1000-3000 Å, and even morepreferably about 2000 Å.

Additional steps according to this method are illustrated in FIGS.8F-8H. According to this embodiment, the protective material 160 isetched back completely to the top surface of the sacrificial material140 such that the protective material 160 is left only in the openings150, as shown in FIG. 8F.

It will be understood that the portions of the sacrificial material 140that are surrounded by the protective material 160 will serve as supportstructures or rails of the device. The skilled artisan will appreciatethat, preferably, the sacrificial material 140, especially the portionssurrounded by the protective material 160, is not harmed when theprotective material 160 is etched because the thickness of thesacrificial material 140 determines not only the height of the supportstructures or posts but also the height of the resonant optical cavity180 (FIG. 8H) that will be created. A change in the height of the cavity180 will affect the color(s) reflected by the device.

The skilled artisan will appreciate that, for example, if the protectivematerial 160 comprises photoresist, a selective etch can be used toselectively remove the protective material 160 from the top surface ofthe sacrificial material 140 without harming the typical sacrificialmaterials 140 (e.g., silicon, molybdenum). Alternatively, an etch-stoplayer can be deposited to protect the sacrificial layer 140. The etchcan also be timed to avoid etching the protective material 160 in theopenings 150. Optionally, a photoresist mask is preferably used to coverthe openings 150 so that the planarization material 160 in the openings150 is not etched away. For example, a reticle can be used toselectively develop the resist in the openings 150 (or resist outsidethe openings 150) and then the protective material 160 is selectivelyremoved from the top surface of the sacrificial material 140.Alternatively, an inverse mask pattern to the mask used to define theopenings 150 can be formed by using the same reticle with a negativephotoresist rather than the positive photoresist used to define theopenings 150, or vice versa. An over- or under-exposure ensures fullcoverage despite small misalignments.

The skilled artisan will appreciate that, alternatively, a backsideexposure process may be used with a negative photoresist mask, resultingin the structure shown in FIG. 8F. According to this embodiment, theprotective material 160, which is preferably formed of a negativephotoresist, in the openings 150 is exposed to light from the backsideof the structure through the substrate 20. The light causes theprotective material 160 in the openings 150 to harden. The portion ofthe protective material 160 over the top surface of the sacrificialmaterial is not exposed to the light because the sacrificial material140 is preferably not transparent, thereby shielding that portion of theprotective material 160 from exposure to the light. The unexposedprotective material 160 on the top surface of the sacrificial material140 is then developed for removal, resulting in the structure shown inFIG. 8F. The skilled artisan will appreciate that, in this embodiment,molybdenum is a preferred material for sacrificial material 140 becauseit is not transparent, whereas silicon is less preferred as a negativephotoresist material because it is semi-transparent. Other preferredmaterials for the sacrificial material 140 include Ti, W, and Ta.

According to one embodiment, the openings 150 have a uniform height toform support structures having a uniform height. In accordance withanother embodiment, the openings 150 have varying heights to formsupport structures having varying heights for a multicolor display. Theskilled artisan will understand that the attached Figures are drawnillustrate the sequence of steps described herein and are not drawn toscale.

After the protective material 160 is etched, a movable layer 170 ispreferably deposited (and subsequently patterned and etched) over thestructure to form the pre-release or unreleased structure illustrated inFIG. 8G. It will be understood that the illustrated embodiment in FIG.8G shows a movable layer 170 deposited over the structure shown in FIG.8F. In the illustrated embodiment, the movable layer 170 functions as amovable reflective layer or second electrode as well as a mechanicallayer, and thus may be referred to as a mechanical layer, a movablelayer, a deformable layer, and/or electrode. The movable layer 170 maycomprise a fully reflective, flexible metal, as shown in FIGS. 7A, 7B,and 8H, or it may support a separate minor, as shown in FIGS. 7C-7E.Other suitable materials for the movable layer 170 include, but are notlimited to, aluminum, aluminum alloys, chromium, and nickel. In otherembodiments, the movable layer 170 may comprise multiple layers, such asAl/SiO₂, Ni/Al/AlO, and Ni/Al/SiO₂. It will be understood that, in themulti-layer embodiments, the movable layer 170 has a reflective layerhaving a backside comprised of a material resistant to the etchant. Themovable layer 170 preferably connects, either directly or indirectly, tothe substrate 20 around the perimeter of the movable layer 170.

After the movable layer 170 is deposited and other steps to complete thedevice (e.g., patterning columns to cross with rows), the sacrificialmaterial 140 is selectively removed. The skilled artisan will appreciatethat the movable layer 170 may also be etched with openings or holes 172so that the etch gas used for sacrificial layer removal can reach thesacrificial material 140. The skilled artisan will understand that theopenings 172 can be etched by masking the movable layer 170 with a mask(not shown) formed of, for example, photoresist, and etching through themask. The mask is removed after the openings 172 are etched. It will beunderstood that, as part of the overall packaging process, theinterferometric modulators are subsequently sealed and protected fromthe environment surrounding the package containing the interferometricmodulators. Preferably, such holes or openings have a diameter as smallas the photolithographic system will permit. In an embodiment, holes oropenings have a diameter in the range of about 2-6 microns. It will beunderstood that a stepper tool may be used to form openings that aresmaller, less than one micron, and more preferably less than 0.5 micron.The skilled artisan will understand that the size, spacing, and numberof openings will affect the rate of removal of the sacrificial material140.

As shown in FIG. 8H, the sacrificial material 140 is removed between thesupports 18 surrounded by the “covers” formed of the protective material160, preferably using a selective gas etching process (e.g., selectiveto the minor or movable layer 170, dielectric 130, and protectivematerial 160), to create the optical cavity 180 between the movablelayer 170 and the dielectric layer 130 of the optical stack 16. Afterselective etching of the sacrificial material 140, the movable layer 170is supported by and rests on the supports 18 formed by the sacrificialmaterial 140.

Various etching processes (including wet methods and dry methods) andetchants may be used to remove the sacrificial material 140. Standardetching techniques well known in the art may be used to remove thesacrificial material 140. Suitable etching techniques include, forexample, wet etching methods and dry etching methods. The particularetching process will depend on the material to be removed.

Chemical dry etching methods typically involve exposure of gaseous,chemically reactive etchants to the sacrificial material 140, convertingthe material into volatile products that are removed, for example, by avacuum source. Examples of etchants useful in dry etching methodsinclude mixtures of one or more gases, for example mixtures of an inertgas (e.g., Xe or Ar) with, for example, N₂, F₂, H₂, CO, Cl₂, N_(x)F_(y)(e.g., NF₃), C_(x)F_(y) (e.g., C₂F₆), and/or Si_(x)F_(y) (e.g., SiF₄).For example, xenon difluoride (XeF₂) produce a dry etchant release gasfor selectively removing a silicon, molybdenum, titanium, or tungstensacrificial layer relative to protective materials 160, including, butnot limited to, silicon oxide, silicon nitride, alumina, photoresist,and aluminum, as XeF₂ does not appreciably etch these materials. It willbe understood that this etching process is a selective etching processthat does not etch the dielectric, semi-reflecting, or electrodematerials, such as the illustrated movable layer 170, protectivematerial 160, and the lower dielectric layer 130, or any etch-stopmaterials over these structures.

An exemplary wet etchant is a phosphoric/acetic/nitric acid or “PAN”etchant, which can selectively remove, for example, Mo, Al, or Gerelative to various materials, including, but not limited to, siliconoxide, silicon nitride, titanium, nickel, chromium, ITO, siliconcarbide, and amorphous silicon. The skilled artisan will appreciate thata sacrificial material 140 comprising tantalum and titanium, forexample, may be removed by either wet or dry etch chemistries,including, but not limited to, etching by etchants, such as XeF₂,fluorine plasma (CF₄, NF₃, SF₆), and Cl₂. A sacrificial material 140comprising magnesium is preferably removed using a wet etch chemistry,including, but not limited to, etching by etchants, such as HCl andHNO₃.

A sacrificial material 140 comprising aluminum is preferably removedusing a wet etch chemistry rather than a dry etch chemistry. Suitablewet etchants for an aluminum sacrificial material 140 include, but arenot limited to, bases such as ammonium hydroxide (NH₄OH) and TMAHorganic base, phosphoric acid, HCl, PAN etchant, NaOH, and KOH. Althougha wet etchant is preferred for removal of aluminum, a dry etchant, suchas Cl₂ plasma, can be used to remove aluminum. The skilled artisan willappreciate that wet etchants used to etch an aluminum sacrificialmaterial may also remove aluminum, if any, on the backside of themovable layer 170 and that any such aluminum on the backside of themovable layer 170 should be protected from the etching with a thin(e.g., <100 Å) dielectric layer. The skilled artisan will appreciatethat, for embodiments in which the movable layer 170 has aluminum on thebackside, nickel (or any metal that is resistant to the etchant) can beused as the reflective material on the backside of the movable layer170, as nickel is resistant to etchants for aluminum.

In some embodiments, the etching is monitored, for example, bymonitoring the reflectivity of the device, or the etching productsreleased. In other embodiments, the etching is conducted for apredetermined period of time. Those skilled in the art will understandthat the etching rate of a layer depends on the thickness of the layer.In general, a thicker layer will etch more slowly than a thinner layer.As described above, the movable layer 170 may also be patterned toprovide openings or holes 172 so that the etch gas used for sacrificiallayer removal can reach the sacrificial material 140. The skilledartisan will understand that the etching rate also depends on the numberof the openings 172 and the size of the openings 172.

In accordance with this embodiment, the resulting final structure of theinterferometric modulator is shown in FIG. 8H. As shown in FIG. 8H, themovable layer 170 on the top surface and the protective material 160 onthe side walls together encapsulate the sacrificial material 140 toprotect it from the release etch that creates the optical cavity 180. Ina particularly preferred embodiment, the sacrificial material 140comprises molybdenum and the final resulting structure of theinterferometric modulator has supports 18 having straight side walls. Inthis embodiment, the protective material 160 may comprise photoresist orSOD.

FIG. 9 illustrates the resulting final structure of an interferometricmodulator fabricated in accordance with an alternative embodiment. Inaccordance with this embodiment, after the sacrificial material 140 isselectively removed from the structure shown in FIG. 8G, the protectivematerial 160 in the openings 150 surrounding the supports are alsoremoved. The selective removal may be performed by any type of selectiveetching process (wet or dry) so long as there is no SiO₂ exposed to theetchant. For example, the skilled artisan will appreciate that an Al₂O₃etch stop over the optical stack 16 could be used or alternativedielectrics for the optical stack 16. Alternatively, an ashing processmay be used to remove the remaining protective material 160 in theopenings 150. The skilled artisan will readily appreciate that theashing process is used to remove protective material 160 comprisingphotoresist.

According to another embodiment, after the protective material 160 isdeposited, as illustrated in FIG. 8E, the protective material 160 isthen etched back to the top surface of the sacrificial material 140 toform a “cover” over certain portions of the sacrificial material 140, asshown in FIG. 10A. Preferably, the protective material 160 is etchedafter masking with a photolithographic process. These portions of thesacrificial material 160 under the “cover” will serve as supports 18(FIG. 10D) in this embodiment. Preferably, the protective material 160is etched back leaving a “cover” slightly wider than the support area toensure covering the support area in case of mask misalignment, asillustrated in the embodiment shown in FIG. 10A. Alternatively, as shownin FIG. 10B, the “cover” is formed as an annulus only over theprotective material 160 in the openings 150. As illustrated by theunreleased structure shown in FIG. 10C, the movable layer 170 isdeposited over the structure after the protective material 160 is etchedto form the “cover.” Although FIG. 10C illustrates the movable layer 170as deposited over the structure shown in FIG. 10A, the skilled artisanwill understand that, in an alternative embodiment, the movable layer170 may be deposited over the structure shown in FIG. 10B. The skilledartisan will understand that subsequent steps of patterning, selectiveremoval, etc., as described above, follow deposition of the movablelayer 170 to complete the device, resulting in the final structure ofthis embodiment, schematically shown in FIG. 10D.

Alternatively, in an embodiment where the protective material 160 isphotoresist, after selective removal of the sacrificial material 140shown in FIG. 10A, an ashing process may be used to remove the remainingprotective material 160 in the openings 150, leaving protective material160 only on the top surface of the sacrificial material 140 in the areafor forming the support structure. The skilled artisan will understandthat an ashing process may be used to remove the remaining protectivematerial 160 in the structure shown in FIG. 10B after selective etchingof the sacrificial material 140.

According to one embodiment, a layer of photoresist 200 may be spun overthe structures shown in FIGS. 10A and 10B prior to deposition of themovable layer 170, as illustrated in FIG. 11A. The removal of theprotective material 160, along with this additional layer of photoresist200, from the top surface of the sacrificial layer 140 results in adevice having an even more planarized movable layer 170. This layer ofphotoresist 200 has a thickness that is preferably on the same order asthe thickness of the protective material 160. Thus, the skilled artisanwill understand that, for example, if the protective material has athickness of about 1000 Å, the thickness of the photoresist layer 200 ispreferably in the range of about 500-2000 Å. According to thisembodiment, the photoresist layer 200 is then patterned and etched sothat it covers only the protective material 160, as shown in FIG. 11B.The photoresist layer 200 and protective material 160 are then bothremoved, preferably by plasma-etching, to yield the structure shown inFIG. 11C. The photoresist layer 200 helps minimize dishing of the topsurface of the protective material 160 within the openings 150 when theprotective material 160 is removed. As shown in FIG. 11C, the movablelayer 170 has a top surface that is only slightly dished in portionsover the openings 150 filled with protective material 160. According tothis embodiment, the movable layer 170 is even more planarized than themovable layer 170 in the embodiment shown in FIGS. 8H and 10D.

In another embodiment, to form a support structure, a self-planarizingmaterial, such as photoresist, can be used to isolate a portion ofsacrificial material until after selective removal of the sacrificialmaterial is performed. FIGS. 12A-12I depict a process for forming adevice comprising such a support structure. As noted above, the skilledartisan will understand that the attached Figures are drawn illustratethe sequence of steps described herein and are not drawn to scale.

In FIG. 12A, it can be seen that a sacrificial material 140 has beendeposited over an optical stack 16, and a mechanical layer, which inthis embodiment is a movable reflective layer 170, has been formed overthe sacrificial material 140. The sacrificial material 140 is depositedto a thickness preferably in the range of about 1000-4000 Å. As shown inFIG. 12A, apertures 322 are etched in the movable reflective layer 170,surrounding a portion 324 of the movable reflective layer 170. Thisportion 324 of the movable layer 170 is positioned over the area of thesacrificial material 140 that will form the support structures. FIG. 12Bis a top view of the partially fabricated device at the stage of FIG.12A. (FIG. 12A is a cross-section of the device taken along line12A-12A) As can be seen in FIG. 12B, the apertures 322 do not completelyisolate the portion 324 of the movable reflective layer 170 from therest of the layer 170.

In FIG. 12C, it can be seen that the sacrificial material 140immediately underneath the apertures 322 has been etched to form acavity 326 surrounding a residual column 328 of sacrificial material140. A timed etch process is preferably used to etch the sacrificialmaterial 140. The etch process which forms the cavity 326 may alsopartially undercut the movable reflective layer 170. The shape of thecavity 326 can be more clearly seen in FIG. 12D, which is a top view ofthe partially fabricated device at the stage of FIG. 12C. The edge ofthe remaining sacrificial material is indicated by a dashed line. As canbe seen, the cavity 326 is an annular cavity surrounding a column 328 ofsacrificial material.

In FIG. 12E, it can be seen that a layer of self-planarizing material330 has been deposited, filling the cavity 326 and extending over themovable reflective layer 170. In an embodiment, the planarizing material330 may comprise a photoresist. The thickness of the planarizationmaterial 330, from the top surface of the movable layer 170 to the topsurface of the planarization material 330, is preferably in the range ofabout 3000 Å to five microns, and more preferably about one micron. InFIG. 12F, the planarizing material 330 has been patterned and etched,and the underlying movable reflective layer 170 has been patterned andetched, forming etch holes 334, which extend through the planarizationlayer 330 and the movable layer 170 to the sacrificial material 140. Itwill be understood that the etch holes 334 are provided for selectiveremoval of the sacrificial material 140. In an embodiment in which theplanarization layer 330 comprises a photoresist, no etching of theplanarization material 330 may be necessary. In the illustratedembodiment, the planarization material 330 within the cavity 326 hasalso been etched, although the column 328 remains insulated by theplanarization material 330, as shown in FIG. 12F. Advantageously, thispatterning will facilitate the later removal of the planarizationmaterial 330, such as by resist ashing.

In FIG. 12G, an etching process is performed to selectively remove thesacrificial material 140 via the etch holes 334. As can be seen, thecolumn 328 of sacrificial material remains, as it is protected from theetch by the planarization material 330. In FIG. 12H, the planarizationmaterial is removed, such as by an ashing process, leaving a column 328of sacrificial material which serves as a support structure. FIG. 12I isa top view of the device of FIG. 12H, in which it can be seen that thecolumn 328 underlies the portion 324 of the movable reflective layer170, providing support for the entire layer.

As the movable mechanical layer 170 was deposited on a planarsacrificial layer, it can be seen that the movable layer 170advantageously remains substantially flat. This substantial flatnessminimizes the likelihood that residual stresses within the layer 170will pull it upwards or downwards from the flat position, and alsominimizes the likelihood that any residual stresses will encouragedelamination, pulling away from the column 328, which serves as asupport structure. Thus, the flat nature of the deposited movable layer170 both minimizes the effect of the residual stresses and the magnitudeof the residual stresses themselves. Furthermore, because the column canbe made substantially cylindrical, the actuation of the movable layer toa collapsed position against the optical stack 16 is less likely toresult in mechanical failure near the edges of the support structure, asmight occur over time in a support structure having unsupported wings.

Those skilled in the art will understand that changes in the device andmanufacturing techniques described above are possible. For example,component and/or steps may be added and/or removed, and the sequence ofsteps may also be altered. Moreover, the methods, structures, andsystems described herein are useful for fabricating other electronicdevices, including other types of MEMS devices and other types ofoptical modulators.

While the above detailed description has shown, described, and pointedout novel features of the invention as applied to various embodiments,it will be understood that various omissions, substitutions, and changesin the form and details of the device or process illustrated may be madeby those skilled in the art without departing from the spirit of theinvention. As will be recognized, the present invention may be embodiedwithin a form that does not provide all of the features and benefits setforth herein, as some features may be used or practiced separately fromothers.

1. A method of making a microelectromechanical systems device,comprising: providing a substrate having a first electrode layer formedthereon; depositing a sacrificial layer over the first electrode layer;forming at least one annular hole in the sacrificial layer; filling theannular hole; depositing a second electrode layer over the sacrificiallayer after filling the annular hole; and removing the sacrificial layeroutside of the annular hole after depositing the second electrode layerto leave a portion of the sacrificial layer that supports the secondelectrode layer above the first electrode layer.
 2. The method of claim1, wherein filling the annular hole comprises depositing a protectivematerial in the annular hole.
 3. The method of claim 2, wherein theprotective material is a self-planarizing material.
 4. The method ofclaim 2, wherein the sacrificial layer is susceptible to etching by anetchant and the protective material is resistant to etching by the sameetchant.
 5. The method of claim 4, wherein the etchant is xenondifluoride.
 6. The method of claim 2, wherein the sacrificial layercomprises an inorganic material.
 7. The method of claim 6, whereinfilling the annular hole comprises depositing a protective material inthe annular hole and the inorganic material is selected from the groupconsisting of silicon, molybdenum, tungsten, and titanium and theprotective material comprises photoresist.
 8. The method of claim 1,wherein the microelectromechanical systems device is an interferometricmodulator device.
 9. A method of making an electromechanical systemsdevice, comprising: providing a substrate having a first electrode layerformed thereon; depositing a sacrificial layer over the first electrodelayer; forming at least one annular hole in the sacrificial layer;filling the annular hole; depositing a second electrode layer over thesacrificial layer after filling the annular hole; and removing thesacrificial layer outside of the annular hole after depositing thesecond electrode layer to leave a portion of the sacrificial layerwithin the filled annular hole, the portion of sacrificial layerconnecting the second electrode layer to the substrate.
 10. The methodof claim 9, wherein filling the annular hole includes depositing aself-planarizing material in the annular hole.
 11. The method of claim9, wherein removing the sacrificial layer includes etching with anetchant and filling the annular hole includes depositing a materialresistant to the etchant.
 12. The method of claim 9, wherein thesacrificial layer is an inorganic material.
 13. The method of claim 12,wherein the inorganic material is selected from the group consisting ofsilicon, molybdenum, tungsten, and titanium.
 14. The method of claim 13,wherein removing the sacrificial layer includes etching with afluorine-based etchant without removing a material filling the annularhole.